Magnesium diboride (MgB2) is a superconducting material having a superconducting transition temperature (Tc) of approximately 39 K. There is a significant interest in using MgB2 to form superconducting wires, tapes, and films. Superconducting wire, for example, can be used for superconducting magnets, fault-current limiters, and power transmission. Films can be used to make Josephson junctions, SQUIDS (superconducting quantum interference devices), micro-electronic interconnects, RSFQ (rapid single flux quantum) devices, and other devices. Films can also be incorporated into RF and microwave devices in the form of resonators, filters, delay lines, and the like.
With respect to film applications, growth of completely in-situ MgB2 films is required in order to realize the multilayer technology necessary for electronics applications. The primary difficulty of depositing MgB2 films is the very high vapor pressures of Mg required for the thermodynamic stability of the MgB2 phase at elevated temperatures. A second problem relating to MgB2 film formation is the high sensitivity of Mg to oxidation. Both of these concerns have made it difficult to grow MgB2 films using conventional physical vapor deposition (PVD) techniques.
In-situ MgB2 films have been fabricated by annealing Mg—B or Mg—MgB2 mixtures in situ in growth chambers. However, films produced by such techniques have shown a lower Tc and poor crystallinity. In-situ MgB2 films have also been fabricated at low temperatures (<350° C.) but these films are not epitaxial, their crystallinity is poor, their Tc values are low, and their resistivities are high. Zeng et al. have grown MgB2 films in-situ by using HPCVD (hybrid physical-chemical vapor deposition) techniques. See X. H. Zeng et al., In situ epitaxial MgB2 thin films for superconducting electronics, Nature Materials 1, pp. 1-4 (2002). However, this method is not readily amenable to multilayer devices or applications requiring large-area film growth. Furthermore, the substrate temperature used in the method proposed by Zeng et al. is above 700° C., and growth has been successful only on a limited number of substrate materials.
There thus is a need for a method of producing MgB2 films in-situ within a temperature range of approximately 300° C. to approximately 700° C. A method is also needed that can grow MgB2 films in-situ on a variety of substrate materials. A method is needed that can produce in-situ MgB2 suitable for use in multilayer device fabrication. The method is also preferably applicable to superconducting films other than MgB2.